Development of a high strength stainless steel with improved toughness and ductility

Development of a High Strength Stainless Steel with Improved Toughness and Ductility

DONALD WEBSTER

Composi t ional modif ica t ions have been made to the ex i s t ing C r - M o - C o s ta in less s t ee l s to p r o - duce a s t ee l (alloy B) which combines the high s t rength of AFC 77 with the toughness of AFC 260. This has been achieved by u t i l iz ing both the s t rengthening effect of grain r e f inemen t and the c rack stopping abili ty of re ta ined austeni te . Af te r t e m p e r i n g at 800 ~ to 900~ alloy B pos- s e s s e s higher elongation than other high s t reng th s t a in l e s s s t ee l s due to the ease with which its re ta ined austeni te t r a n s f o r m s under s t r e s s to m a r t e n s i t e to delay necking. An explanation has been advanced for the anomalously low tens i le y ie ld s t rength that occu r s in both alloy B and AFC 77 af ter t emper ing at 1000~

T HE highest s t reng th s ta in less s t ee l s contain as m a j o r a l loying e l emen t s carbon, ch romium, cobalt , and molybdenum. These s tee l s , which are typified by AFC 77 and AFC 260, a re des igned bas ica l ly as a l loys for use at e leva ted t e m p e r a t u r e s . The composi t ions of AFC 77 and AFC 260 used in this paper a re given in Table I.

Although these s t ee l s were not designed for opt imum toughness , exce l len t combinat ions of s t rength and toughness can be obtained by ca re fu l se lec t ion of the aus teni t iz ing t e m p e r a t u r e ~ which cont ro ls the amount of r e t a ined austeni te p re sen t a f te r heat t r ea tmen t . AFC 260 p o s s e s s e s good toughness up to its max imum s t rength but is l imi t ed to a t ens i l e s t rength of 265 ksi . AFC 77 can be heat t r ea t ed to s t rengths of up to 290 ks i by t emper ing at 900 ~ to l l 0 0 ~ but its toughness is l imi t ed by the sma l l amount of re ta ined austeni te that can be mainta ined at these high t e m p e r i n g t e m p e r a - tu res . The object of this work was to produce a s t a in - l e s s s t ee l with p r o p e r t i e s supe r io r to AFC 260 and AFC 77 by minor compos i t iona l va r i a t ions designed to take advantage of the p rev ious ly demons t ra t ed bene- f i ts of re ta ined aus ten i te ' and fine austeni te gra in sizefl

EXPERIMENTAL TECHNIQUE

M a t e r i a l

Two 100 lb heats of v a c u u m - m e l t e d m a t e r i a l were hot ro l l ed at 2100~ into 4 by 0.6 in. s labs. The f i r s t heat, alloy A, was des igned to t es t the grain ref ining ef fec t of niobium alone. The second heat, alloy B, was des igned to tes t the cumula t ive ef fec ts of grain r e f ine - m e a t and the aus teni te s tab i l iza t ion produced by 1 pct NL The chemica l composi t ion (wt pct) of the heats a re shown in Table II.

Heat T r e a t m e n t

Austeni t iz ing t r e a t m e n t s were for 1 hr at 1700 ~ 1900 ~ 2100 ~ and 2200~ Specimens aus ten i t ized at 2100 ~ and 2200~ w e r e cooled to 1900~ and held for 1 hr before oi l quenching to allow t ime for r e s idua l

DONALD WEBSTER is Senior Engineer, Materials Research Group, Boeing Company, Seattle, Wash.

Manuscript submitted October 26, 1970.

6 f e r r i t e to t r a n s f o r m to austeni te , s Af te r quenching to r o o m t e m p e r a t u r e spec imens were then cooled to - 1 0 0 ~ for 1 hr be fore t emper ing in the range 500 ~ to 1200~ for 2 + 2 hr. (2 hr at t e m p e r a t u r e followed by an a i r cool and then a fu r the r 2 hr at t empera tu re . ) Some spec imens were given an in t e rmed ia t e 700~ t e m p e r before t e m p e r i n g at h igher t e m p e r a t u r e s . In some cases double aus teni t iz ing t r e a t m e n t s were used in which the s amples were f i r s t aus ten i t ized at 1700~ or 2200~ and then aus ten i t i zed in the range 1900 ~ to 2200~ These double t r e a t m e n t s were designed to vary austeni te gra in s ize .

Mechanica l Tes t ing

The techniques used to de t e rmine f r ac tu re toughness , t ens i le , and c o m p r e s s i v e p r o p e r t i e s on the 0.6 in. plate have been prev ious ly d e s c r i b e d 1 and need not be r e - peated here except to say that al l spec imens were t e s t ed in the longitudinal d i rec t ion and the t ens i l e elongation values were m e a s u r e d on a 1 in. gage length.

De te rmina t ion of Retained Austeni te

The amount of re ta ined austeni te p re sen t in the t e s t s p e c i m e n s a f te r heat t r e a t m e n t was de t e rm ined by an improved X - r a y technique that is des igned to e l imina t e the effects of p r e f e r r e d or ien ta t ion which is p r e s e n t in some samples . In this method the m a r t e n s i t e (211)

Table I. Chemical Composition of AFC 77 and AFC 260

Cb Alloy C Si Mn S P Cr (Nb) Mo Co Hi N

AFC77 0.16 0,13 0.18 0.021 0.015 14.0 - 5.02 13.41 0.21 0.04 AFC260 0.08 0.30 0.34 0.012 0.015 15.42 0.14 4.30 13.03 1.94 0.02

Table II. Chemical Composition of Alloys

Cb Alloy C Si Mn S P Cr (Nb) Mo Co Hi R

A 0.14 0,05 0.18 0.012 0,015 14.12 0.13 5,08 t3.45 - 0.060 B 0.16 0.06 0.20 0,017 0.018 13.94 0.22 5.22 13.67 1.03 0.032

METALLURGICAL TRANSACTIONS VOLUME 2, AUGUST 1971-2097

peak and the austeni te (220) peak a re s t ep - scanned at a r a t e of 0.1 deg per min while the spec imen is both ro ta ted and ti l ted.

Measuremen t of Austeni te Grain Size

The austeni te gra in s ize was m e a s u r e d as the mean l i n e a r in tercept of 500 gra ins . M e a s u r e m e n t s were made opt ical ly for grain in te rcep t s over 20 ~. Gra in i n t e r cep t s under 20 ~ were de t e rmined by the e x a m i n a - tion of r ep l i cas in the e lec t ron m i c r o s c o p e .

RESULTS

Austeni te Grain Size

In the ini t ial heat t r e a t m e n t t e s t s , spec imens aus - t en i t i zed at 2200~ were found to be c o a r s e gra ined in both a l loys in spite of the p r e s e n c e of niobium.

M a t e r i a l which had been aus ten i t i zed at 2200~ was then r e a u s t e n i t i z e d at l ower t e m p e r a t u r e s but was found to be c o a r s e g ra ined at a l l t e m p e r a t u r e s , Fig. 1. Alloy B had a duplex gra in s t r u c t u r e at mos t t e m p e r a - tu res . The r e l a t i ve ly fine gra in reg ion between 1900~ and 2000~ obse rved for s tee l A is an abnormal i ty f r e - quently obse rved when sma l l s p e c i m e n s containing un- d i s so lved ca rb ides a r e rapid ly heated. 4'5 It is thought to be due to vary ing d e g r e e s of d iscont inuous gra in growth. Slow heat ing r a t e s which s imula t e the heat ing of l a r g e r components produce a p r o g r e s s i v e l y c o a r s e r gra in s ize as the a u s t e n i t i z i n g t e m p e r a t u r e is i nc rea sed . A s e r i e s of e x p e r i m e n t s d e m o n s t r a t e d that the f ines t aus ten i te gra in s ize was produced f r o m annealed ma te ~ r i a l which had been given an aus ten i t i z ing t r e a t m e n t at 1700~ for 1 hr, Fig. 1. Af te r th is t r e a t m e n t al loy B is somewhat f iner gra ined than A, probably because of the h igher niobium content of the f o r m e r alloy.

120

20

I O0

60

,~ 0o

~ 40

25

140

_ _ A 1500

I

r

DUPLEX GRAIH ~ \ ~. SIZE

NN

1700

ALLOY

1900 2100

ALISTEMITIZ ING TEMPERATURE (OF)

PRIOR AUSTENITIZING TEMP., ~ 1700 2200

0 O

2300

Fig. 1--Effect of austenitizing temperature and prior treatment on austenite grain size of alloys A and B.

1300

20 .--,,-" ~ , -

,,c

15

5

O

5OO

- - - ALLOY B �9 �9

700 900 I00

TEMPERING TEMPERATURE (oF)

Fig. 2--Effect of austenitizing and tempering temperatures on the amount of retained austenite in alloys A and B.

2098-VOLUME 2, AUGUST 1971 METALLURGICAL TRANSACTIONS

Fig. 3--Effect of austenitizing and tempering temperatures on the tensile strength of alloy A.

300

250

200

z

150

100 500 600

Control of Retained Austenite

It has been shown previously I that for AFC 77 the amount of retained austenite increased with austenitizing temperature and decreases with tempering temperature. Similar behavior for alloys A and B austenitized at 2100 ~ and 2200~ is shown in Fig. 2. This figure shows that for equivalent treatments the alloy B contains more retained austenite than alloy A at all tempering temperatures indicating that the basic ob- jective of the nickel addition was achieved. The increase in the amount of austenite between 1150 ~ and 1200~ in alloy B may indicate that the nickel lowered the Acl to within this temperature range.

Mechanical P r o p e r t i e s

The tensi le p r o p e r t i e s of al loy A are shown in Fig. 3. The p rope r t i e s as expected a re s i m i l a r to those r e - por ted for AFC 77.' The usual drop in yield s t rength a f te r t emper ing at 1000~ is obse rved af te r aus ten i t i z - ing at 2200~ but not after austenitizing at 1900~ A significant increase in strength due to grain refine- ment is apparent when the fine grained material austenitized at 1700 '~ and then 2100~ is compared with the very coarse grain material austenitized at 2200~

The highest yield strengths in alloy A are obtained by austenitizing at 1900~ which is probably due to both the very fine grain size and the low retained austenite content in this condition.

The toughness and ductility of alloy A are shown in Fig. 4 and again these are similar to previously re- ported results on AFC 771 where toughness increases with austenitizing temperature. Elongation is not markedly dependent on austenitizing temperature but reduction of area increases as the austenitizing temperature increases

The tensile properties of alloy B, Fig. 5, are essen- tially similar to those of alloy A except that the tensile and yield strengths of B are lower at tempering temperature below 800~ and there is a more marked drop in yield strength after tempering at 1000~ The tough-

ALLOY A AUST. TEMP.

I ~

1700 + 1900 1700 + 2100

2200

0.2%1Y. S, UT$ 0

�9 0

1oo

~ 50

0

20

- I0 .J

700 000 900 1000 1100 1200

TEMPERING TEMPERATURE (OF)

ALLOY A AUSTENITIZINGI700 +1700+ 220021001900 TEMPo0~ OF

1700+ 2200 �9 I COARSE GRAIN

60

40

20

000 700 000 900 1000 1100 1200

TEMPERING TEMPERATURE (OF) Fig. 4--Effect of austenitizing and tempering temperatures on the toughness and ductility of ahoy A.

hess and tensile elongation of alloy B, Fig. 6, show improvements over alloy A at all tempering temperatures with optimum elongation values being obtained at 800~ It was found that some increase in the elongation of alloy B could be obtained if an initial temper at 700~ was given before the final temper at higher tempering temperatures, Fig. 7. However, this increased ductility which is apparently due to austenite


w

cD

z

z

300

250

200

150

100 e,

500

I

AUST. TEMP. o F

ITO0 + 1900 1700 + 2100

2200

[

0.25 Y. S.

|

ALLOY 8

I

I 600 700 900 900 1000 11 O0 1200

I

UTS

o D


FINE GRAIH FINE GRAIN COARSE GRAI[

Fig. 5--Effect of austenitizing and tempering temperatures on the tensile strength of alloy B.

1 0 0

c:

'p,

v 50

25

29

: 15 ..-,.

10

0

0

60

4O

"~ 20 ,..:

ALLOY 0 AUST. TENP.,~ I 1700+ 1900 o FINE GRAI~

1700+ 2100 o FINE GRAI~ 2200 A COARSE

.. . - - - - ~ 1 "~l - '~,2

1

500 600 700 800 900 1000 1100 1200 TEMPERING TEMPERATURE (OF)

Fig. 6--Effect of austenitizing and tempering temperatures on the toughness and ductility of alloy B.

s tabi l iza t ion at 700~ is obtained with some sacr i f ice in s t rength.

Charpy V notch tes ts were conducted on samples of alloy B which had been aus ten i t i zed at 2100~ and t empered at 800~ Omis s ion of the subzero t r ea t - men t before t emper ing r e s u l t e d in a large drop in yield s t rength and r a i s ed the impact energy s igni f i - cantly as shown in Table III.

The c ompr e s s i ve p rope r t i e s of a l loys A and B are shown in Fig. 8. In genera l there is an i n c r e a s e in yield s t rength and propor t iona l l imi t with i n c r e a s e d t emper ing t e m p e r a t u r e . The very low yield s t reng ths observed in tens ion af ter t e m p e r i n g at 1000~ a re not observed in compress ion .

The effect of cold working on a l loy B in i ts mos t duct i le condition (austeni t ized 2100~ quenched to - 1 0 0 ~ and t empered 800~ was examined by cold ro l l ing 0.080 in. sheet . Cold reduc t ions up to at l eas t 68 pct could be obtained without edge-c rack ing . The r e su l t an t mechan ica l p rope r t i e s a re shown in Fig. 9. The re is a s ignif icant i n c r e a s e in both yield and t en - s i le s t rength as a r e su l t of cold working. Aging at 800~ after the working produces a fu r the r i n c r e m e n t of s t rength. The elongation drops rapidly with cold work but duct i l i ty as m e a s u r e d by reduct ion of a r e a r e m a i n s at a high level for a m a t e r i a l of this s t rength .

DISCUSSION

The compositional modifications made to AFC 77 to produce alloy B were designed to refine the austenite gra in size and i nc r ea se the s tab i l i ty of the aus ten i te a f te r heat t r ea tment . The benef i t s of aus ten i te g ra in r e f inemen t have prev ious ly been shown to be s ign i f i - cant for s tee ls in this composi t ion. 2 The benef i t s in - clude i n c r e a s e s in s t rength and s t r e s s c o r r o s i on r e - s i s t ance and de c r e a se s in fatigue c rack growth ra te . A niobium addition is a convenient way to ref ine aus - teni te gra in size although the r e s u l t s show that for ma x i mum gra in r e f inemen t the n iob ium carbide d i s - pe r s ion has to be opt imized by a p r e l i m i n a r y a u s - t en i t iz ing t r ea tmen t before the f inal heat t r e a t m e n t .

The improvement in f r ac tu re toughness in AFC 77 produced by sma l l amounts of r e t a ined aus ten i te a re subs tan t ia l and high magnif ica t ion c i n e - f i l m s of c racks growing in AFC 77 sheet have d e m o n s t r a t e d that aus - tenite is an effective crack s topper . Cracks growing through m a r t e n s i t e were a r r e s t e d on r each ing an a r e a of aus teni te and then under i n c r e a s i n g load were seen


to branch and grow around the a r e a of austeni te . In AFC 77 this mechanism is mos t ef fec t ive in the s t rength range 240 to 260 ksi produced by t e m p e r i n g at 500 ~ to 800~ At the higher t emper ing t e m p e r a t u r e s (800 ~ to l l00~ needed to obtain s t reng ths in the range 260 to 290 ksi, the amount of aus teni te that can be re ta ined de- c r e a s e s markedly and r educes the e f f ec t iveness of the

300 I

~250 . IY" ~ ] TENSILE STD~gTH

I I \ \ l a ALLOY 0 I '0STEM'T,ZEO .00o. ,100o I J . TEMPERED DIRECTLY -

100[ J 0 DOODLE TEMPER (70OAF 2 HR+ INDICATED TEHP.)

o

. 50 I 40

t

Pretempered - - 0 / < 700~ C-

900 900 1000 1100


Fig. 7--Effect of pretempering at 700~ on the strength and ductility of alloy B.

technique. The nickel addition in alloy B is designed to s tab i l ize the aus teni te and allow more to be re ta ined at high t e m p e r i n g t e m p e r a t u r e s . AFC 260 is a/so a mod- i f icat ion of AFC 77 produced by adding nickel (1.85 pct) and niobium (0.15 pct) but in this case the carbon content is lowered f rom 0.15 pct to 0.07 pct which r e - sul ts in a l ower s t rength and tends to negate the austeni te s tab i l i z ing effect of the nickel.

Af te r t e m p e r i n g at 800~ alloy B p o s s e s s e s unusual ly high elongat ion for an aLLoy of this s t rength. This elongation is not sole ly r e l a t ed to the amount of r e - tained austeni te s ince m a t e r i a l t empe red at lower t em- p e r a t u r e s and containing m o r e re ta ined aus teni te has a lower t ens i l e elongation. The high elongation is thought to be r e l a t ed to the s tabi l i ty of the re ta ined aus teni te under applied s t r e s s . An examinat ion of the tens i le s t r e s s - s t r a i n d i a g r a m s for alloy B shows that a l a rge pa r t of the total elongation occurs before the m ax im um load is r eached ; i . e . , it is uni form e longa- tion. M i c r o s t r u c t u r a l examinat ion of de fo rmed specimens indica tes that a s t r e s s - i n d u c e d m a r t e n s i t e t r ans fo rma t ion is occur r ing . The rapid w ork -ha rden - ing ra te produced by the s t r e s s - i n d u c e d t r a n s f o r m a - t ion of aus teni te to m a r t e n s i t e is well known as a means of producing a high uni form elongat ionf l

An unusual effect obse rved in AFC 77 is an anomalously low tens i l e y ie ld s t rength a f te r t e m p e r i n g at 1000~ ~'8 The s a m e effec t is obse rved in alloy A, Fig. 3, and is even m o r e pronounced in al loy B, Fig. 5. T h e r e is no discontinui ty a f te r a 1000~ t e m p e r in o ther mechan ica l p r o p e r t i e s such as t ens i l e s t rength, hardness , f r a c t u r e toughness , or ducti l i ty. Kasak e t

a l . 8 have inves t iga ted the yield point anomaly and con-

Table I lL Tensile and Impact Values of Alloy B Tempered at 800~

0.2 pct Y.S., UTS, Charpy,

Heat Treatment ksi ksi ft-lb

1700~ + 2100~176 800~ 200 258 34 1700~ + 2100~ 800~ 40 238 96

Fig. 8--Effect of austenitizing and tempering temperature on the compressive strength of alloys A and B.

275

250

225

~ 200

175

150

125

COMPRESSIVE PROPERTIES OF ALLOYS A AND B

AUSTENITIZING TEMP., OF A R

1TO0+ 1900 o �9 I ' 2200 ~. �9

I 700 000 900 I000

PROPORTIONAL LIMIT



cluded that it was probably related to two overlapping precipitation reactions. They acknowledge however that the failure os the ultimate tensile strength and hardness to show a similar dip after tempering at 1000~ does not support this type of mechanism. The possibility that austenite reversion during aging at 1000~ is responsible for a dip in yield strength was

400

v

=350

_~3~0

": 250 ,,:.

2001

1 I I I

~ ~ ~ ,, ' J [_ca ~

ALLiY B ~ / ~ ~ /

~/. , I o.~+ Y.s. /'

/'/ 1

3I

H

\ \

EL., ~i F"-'-.-.-

I

""-----.-o

tO 20 30 40 50 60 70 PERCENT REI]UCTtON

Fig. 9--Effect of cold reduction by ro l l ing on the s t reng th and ducti l i ty of alloy B which has been aus teni t ized at 2100~ and tempered at 800~

r e j e c t e d by t h e s a m e a u t h o r s p a r t l y b e c a u s e X - r a y m e a s u r e m e n t s s h o w e d a c o n t i n u o u s d e c r e a s e in a u s - t e n i t e c o n t e n t f o r s a m p l e s t e m p e r e d in the r a n g e 900 to 1300~F. E v i d e n c e p r e s e n t e d by K a s a k e t a l . ~ s h o w i n g a m a r k e d c o n t r a c t i o n d u r i n g t e m p e r i n g a t 1000~ is used below to form the basts for a model which ap- pears to explain many features of the anomalous yield point.

The length and strength changes measured by Kasak et al. ~ during tempering of AFC 77 austenitized at

W:

300

2~o i

20[

15~

AFC 77 AUSTENiTIZED 2000~ O.U.-1OO~ ~ HR.

X X x, I " %.

TENSILE STRENGTH / \ / \ . s x

X

0.25 YIELO STRENGTH

i | I i I I

Q AUSTENITE [] MARTENSIT;

2O0

N

IOO

o - ~

-IOO

I I I zoo .oo . o s~o ,~oo ,~oo '

TERPERiNG TEMPERATURE (OF)

Fig. 10--Changes in length and tensile strength during the tempering of AFC 77. (Kasak et al. 6)

|

AI~TT AFC 260 0 AIX~y A [D ALI~y B �9

12.5

100

o.. AFC2~" \

".o. ~..,_ ~t~Y

LP-a'IgATE TmlSILE S~TH (KSI)

Fig. l l - - C o m p a r i s o n of toughness and ten- s i le s t reng th of al loys A and B with ex i s t - lag s ta in less s tee l s .

2t02-VOLUME 2, AUGUST 1971 METALLURGICAL TRANSACTIONS

2000~F and cooled to-100~F are shown in Fig. 10. In the untempered condition there is 20 to 30 pct austenite existing as a coarse dispersion between the martensite needles. As the martensite is less dense than the austenite from which it forms it exerts a compressive stress on the austenite. A further compressive stress is exerted as precipitation reactions cause the martensite to expand during tempering in the range 700 ~ to 900~ The change in retained austenite content in this range is too small to account for the observed expansion.

During tempering at 1000~ the martensite contracts so that a tensile stress is exerted on each region of austenite. Tempering at higher temperatures up to 1300~ again causes expansion of the martensite and the compressive stresses on the austenite return. The above sequence of events will produce a yield strength dip after tempering at 1000~ as the tensile stress existing in the austenite will allow plastic deformation to occur at a lower than normal applied tensile stress. Between yield stress and ultimate stress the majority of the retained austenite will transform to martensite so that no drop in ultimate tensile strength will occur. On the basis of the above model the following predic- tions can be made regarding the effects of heat treatment, composition, and change of stress direction:

I) The compressive yield strength will not show a dip after a 1000~ temper. In compression the tensile stresses will require a higher than normal applied load so that a peak in compressive strength after a 1000~ temper should be observed. Fig. 8 shows that in compression both the yield strength and the proportional limit tend to increase as the tempering temperature is raised from 900 ~ to 1000~ Similar behavior is observed for AFC 77. A further point of confirma- tion can be seen in the results of alloys A and B austenitized at 1900~ and tempered at 1000~ Alloy A which contains little retained austenite has the highest strength in tension but the lowest strength in compression.

2) The yield strength dip will only occur in samples

which contain significant amounts of retained austenite. In AFC 77, alloy A and alloy B, the amount of retained austenite is decreased by decreasing the austenitizing temperature so that the yield strength dip should not occur if the austenitizing temperatures are too low to allow austenite retention after a 1000~ temper. Data on AFC 776 shows that a low yield strength after a 1000~ temper occurs only after austenitizing at 2000~ and not after austenitizing at 1800 ~ and 1900~ Alloys A and B show this same effect, Figs. 3 and 5, with the dip being more pronounced after austenitizing at 2100 ~ to 2200~ than at 1900~ Further confirma- tion of this effect is the fact that the loss of yield strength is more pronounced in alloy B which has a higher retained austenite content than alloy A. The phenomenon is not confined to steels of the AFC 77 type as evidenced by data on a series of 12 pct Cr, 2 pct Ni, 1~ pct Mo steels investigated by Irvine et al.7 These workers found an anomalously low yield point after tempering at 550~ (1022~ when the carbon content of the series was raised from 0.12 to 0.16 pct to 0.20 to 0.26 pct, an increase which undoubtedly causes austenite retention.

One of the aims of this program was to improve the strength-toughness combinations available in com- mercial steels. The extent to which this has been achieved can be judged from Fig. II which compares alloys A and B with the existing stainless steels. 7 The steepness of the fracture toughness-strength re- lationship for AFC 77 and alloy A is a reflection of the diminishing austenite content at the higher strength levels. In AFC 260 the austenite is maintained at a high level up to the maximum strength of 265 ksi so that the strength-toughness line is considerably less steep than for AFC 77. Alloy B which retains austen- ire up to an ultimate strength of 290 ksi has a strength- toughness line which is a continuation of the AFC 260 line.

A comparison of the tensile strength-elongation combinations of alloys A and B with those of the cur- rently available stainless steels 9 is shown in Fig. 12.

Fig. 1 2 - - C o m p a r i s o n of e l onga t i on v a l u e s of a l l oys A and B with e x i s t i n g s t a i n l e s s s tee Is.

35

I7 2o

10

~o ALLOY B

~I.DY A ~ --~ & X-15 ~ 6 __--rl [3

'///////Xi ~ o

175 200 J , , i

225 250 275 3OO

ULTIMATE TI~SILE STR~GTH (KSI)


Al l t he s t e e l s u s e d in t h i s c o m p a r i s o n w e r e t e s t e d u n d e r i d e n t i c a l c o n d i t i o n s . E l o n g a t i o n v a l u e s in a l l c a s e s w e r e m e a s u r e d on a 1 in. g a g e l e n g t h . T h e a l - l o y s p l o t t e d o u t s i d e the s h a d e d a r e a a r e s i m i l a r in c o m p o s i t i o n and c o n t a i n 13 to 20 p c t Co. T h e h i g h e l o n g a t i o n of X - 1 5 i n d i c a t e s t h a t e v e n w i t h o u t a u s t e n i t e t he c o m p o s i t i o n of the a l l o y s o u t s i d e t he s h a d e d a r e a i s b a s i c a l l y a d u c t i l e one . The a u s t e n i t e s t a b i l i t y u n d e r s t r e s s a n d h e n c e the w o r k - h a r d e n i n g r a t e d e c r e a s e s in the o r d e r of A F C 260, A F C 77, and a l l oy B, and i s r e - f l e c t e d in the i n c r e a s i n g e l o n g a t i o n v a l u e s .

CONCLUSIONS

1) T h e a d d i t i o n of 0 .13 to 0 .22 pc t Nb to s t a i n l e s s s t e e l s of t h e A F C 77 t ype can r e s u l t in m a r k e d a u s - t e n i t e g r a i n r e f i n e m e n t .

2) T h e a d d i t i o n of 1 p c t n i c k e l to A F C 77 r e s u l t s in an i n c r e a s e in the r e t a i n e d a u s t e n i t e c o n t e n t a t h i g h t e m p e r a t u r e s (800 ~ to l l 0 0 ~

3) An a l l oy (B) d e s i g n e d to t a k e a d v a n t a g e of b o t h t h e g r a i n - r e f i n i n g e f f e c t s of n i o b i u m and the c r a c k - s t o p p i n g a b i l i t y of r e t a i n e d a u s t e n i t e p r o d u c e d by n i c k e l p o s s e s s e s a c o m b i n a t i o n of s t r e n g t h a n d t o u g h - n e s s s u p e r i o r to t h a t of e x i s t i n g s t a i n l e s s s t e e l s .

4) Al loy B p o s s e s s e s h i g h e r - t h a n - n o r m a l e l o n g a t i o n when t e m p e r e d a t 800 ~ to 900~ T h i s i s t h o u g h t to b e due to t he e a s e w i th w h i c h the s t r e s s - i n d u c e d a u s t e n i t e to m a r t e n s i t e t r a n s f o r m a t i o n can o c c u r to d e l a y n e c k - ing.

5) T h e a n o m a l o u s l y low t e n s i l e y i e l d s t r e n g t h o b - t a i n e d in A F C 77 t ype a l l o y s a f t e r t e m p e r i n g a t 1000~ i s due to r e s i d u a l t e n s i l e s t r e s s e s in t he a u s t e n i t e .

RE FERENCES

1. D. Webster: ASM Tranz Quart., 1968, vol. 61, p. 816. 2. D. Webster: ASM Trans. Quart., 1969, vol. 62, p. 759. 3. D. Webster:Met. Tranz, 1971, vol. 2, pp. 1857-62. 4. D. Webster: J. Iron SteelInst., July 1962, vol. 200, p. 520. 5. D. Webster: Boeing Document D6-25220, April 1970. Available from DDC. 6. A. Kasak, V. K. Chandhok, ~l. H. Moll, and E. J. Dulls: Development o f High

Strength, Elevated Temperature, Corrosion Resistant Steel. Tech. Doe. Rept. ASD-TDR-63-766, Sept. 1963. Prepared under USAF Contract No. AF 33(616)-7376.

7. K. J. Irvine, D. J. Crowe, and F. B. Picketing: J. Iron Steellnst., 1960, vol. 195, p. 386.

8. V. F. Zackay, E. R. Parker, D. Fahr, and R. Busch: ASM Tranx Quart., 1967, vol. 60, p. 252.

9. C. S. Carter, D. G. Farwick, A. M. Ross, and J. M. Uchida: Corrosion, 1971, vol. 27, no. 5, p. 190.


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Development of a high strength stainless steel with improved toughness and ductility